As is well known in the aerospace industry art, an increasing number of articles of manufacture are made of composite materials, such as propeller blades for modern aircraft and radomes that house radar emitting devices. Manufacture of such articles requires great care to ensure proper performance. Typically, non-destructive testing of the articles is undertaken by means of immersion ultrasonic inspection.
For example, a composite propeller blade may include an exterior airfoil surface shell bonded to an interior composite laminate spar. It is critical that the bond interface between the shell and spar laminate has no flaws, and that the thickness of the propeller blade assembly conforms to design specifications after the bonding process. Neither characteristic can be visually determined, so immersion ultrasound inspection affords a determination of both the quality of the bond and the consistency of the propeller blade thickness.
Known immersion ultrasound test equipment typically involves a test tank that includes a mounting fixture to secure the workpiece to be tested under an ultrasound transmission medium such as water. A bridge assembly supports an ultrasound transducer mount, and enables the mount to traverse both a longitudinal and transverse axis of the secured workpiece surface so that a transducer within the mount can scan the surface. A transducer position controller controls the bridge assembly and ultrasound transducer mount, and standard ultrasound transducer instrumentation controls signals generated by the transducer, and processes, plots and/or stores information received by the transducer.
In performing an inspection of a workpiece, the transducer position controller directs the bridge assembly and transducer mount to commence a scan of the workpiece in a first direction, parallel to its longitudinal axis, starting at a first exterior lateral edge of the workpiece at a base end and proceeding toward a top end of the workpiece. When that scan is completed, the controller directs the bridge assembly and transducer mount to index the transducer in a direction parallel to the transverse axis of the workpiece a specific increment of length, or index distance, away from the first exterior edge of the workpiece. Next the controller directs the bridge assembly and transducer mount to commence a scan in a second direction opposed to the first direction, parallel to the longitudinal axis of the workpiece, starting at the top and going toward the base end of the workpiece. When that scan is complete, the controller again directs an indexing of the transducer in a direction parallel to the transverse axis away from the exterior edge the same index distance, and initiates another scan in the first direction. The inspection process is repeated until the controller has indexed the transducer entirely across the workpiece to a second exterior edge of the workpiece, opposed to the first exterior edge. During the inspection, the transducer instrumentation processes signals received from the transducer to measure the thickness of the workpiece, or a specific portion thereof, and to detect any internal flaws in the workpiece, in a manner well known in the art.
During ultrasound longitudinal wave testing, virtually all known immersion ultrasonic transducers must be at ninety degrees to the workpiece surface with respect to both its longitudinal and transverse axes and the transducers must remain at a constant distance to the workpiece surface, which conditions are otherwise referred to herein as keeping the transducer normal to the workpiece surface. In the circumstances described above, and as is common with most composite materials needing immersion ultrasonic inspection, the workpiece surface is non-planar, and typically involves differing contours for each different workpiece, such as an elongate, curvilinear propeller blade or a cone-shaped radome. Therefore, the transducer mount must somehow enable the transducer to remain normal to the workpiece surface as the workpiece is inspected.
Many approaches to this requirement have been made, such as that described in U.S. Pat. No. 3,575,043 to Allen, which patent is incorporated herein by reference. In Allen, and in many subsequent commercial applications, the transducer is maintained normal to the surface through an articulating control mechanism that affords movement of the transducer in its three axes of potential movement (for purposes of convenience referred to herein as the longitudinal, transverse, and vertical axes). The articulating control mechanism positions the transducer in response to information loaded into the mechanism that defines the surface of the workpiece. Initially, as in Allen, such information was integrated with the control mechanism through a tape-programming assembly. More modern mechanically encoded systems utilize condensed information transfer methods, yet these systems involve enormous labor costs, as a transducer positioning means must typically be manually re-set in at least one of the three axes after each scan, or at frequent intervals during the inspection, as well as between different workpieces.
More recently, workpiece surfaces have been defined digitally, so that computerized controllers can decrease overall operating times by affording transducer positioning normal to the workpiece surface throughout the entire inspection without the need for manual re-positioning during the inspection. Typical of these computerized immersion ultrasound inspection systems is the "Multiscan Precision Scanning System", Model No. PASAD-MS-1660-03, manufactured by Panametrics, Inc., of Waltham MA 02154. These systems still require specific information defining the surface of each workpiece, so that the system must be re-configured for every different workpiece. Such systems require a digital definition of the workpiece surface mandating substantial investment of time and money prior to an inspection, especially where the workpiece is an older structure that may have been designed before the advent of "Computer Assisted Design", or CAD systems; or where the workpiece is damaged, so that its surface no longer conforms to its designed contours. Additionally, because of the many complexities of the computer-based transducer position controlling means, such systems are very costly compared to the aforesaid manually encoded mechanical test systems.
Accordingly it is the general object of the present invention to provide a mechanical contour follower for positioning an ultrasound transducer that overcomes the problems of the prior art.
It is a more specific object to provide a mechanical contour follower that positions a transducer normal to a workpiece surface throughout an immersion ultrasound inspection that does not require mechanical or computerized re-positioning during the inspection.
It is another specific object to provide a mechanical contour follower that positions a transducer normal to a first workpiece surface and to a second workpiece surface that does not require mechanical or computerized re-positioning between immersion ultrasound testing of the first and second workpieces.
It is yet another object to provide a mechanical contour follower for transducers that overcomes the financial and labor cost problems of transducer positioning means in known immersion ultrasound systems.
The above and other advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.